Radioactive composition



United States Patent 3,230,176 RADIOACTIVE COMPOSITION Herbert Edward Farmer, Lynn, and Arthur Hever Buckley, Salem, Mass, assignors to General Electric Company, a corporation of New York No Drawing. Original application Oct. 23, 1958, Ser. No. 769,152, now Patent No. 3,078,995. Divided and this application Oct. 3, 1961, Ser. No. 144,004

2 Claims. (Cl. 252301.1)

This is a divisional application of application Serial Number 769,152, filed October 23, 1958, now Patent Number 3,078,995, and assigned to the assignee of this application.

The present invention relates to a radioactive compo.- sition and, more particularly to magnetic radioactive composition useful in radioactive magnetic particle inspection.

In the manufacture of many articles of magnetic materials, rigid inspection is essential to maintain high standards of quality'and flaw detection is an important phase of such rigid inspection.

Nondestructive-tests are designed to indicate surface or near-surface or defects without destroying or damaging the article being examined. Such tests are used primarily on articles designed for hazardous or critical service or having a high finished value for which destructive tests would be too expensive. In one such nondestructive test known as magnetic particle inspection, magnetic particles are applied to an article in which a magnetic field has been induced. The particles are then attracted to a surface or near-surface defect or flaw existing in the article. An accumulation of particles which occurs at such a defect because of magnetic flux distortion and resulting leakage field at that point makes possible a noticeable indication proportional to the extent of the defect. Proper use of magnetic particle inspection requires considerable experience by human operators in the interpretation of such accumulations or defect indications and the evaluation of defects. Experience has shown that two major limitations of conventional magnetic particle inspection of mass produced articles are that ordinary visual methods require individual interpretation dependent on the skill of a human operator and that such operator in looking at a predominance of satisfactory articles tends, as he fatigues, to approve some articles which are actually unacceptable.

An object of our invention is to provide radioactive magnetic particle materials which will provide detectable contrasts between the accumulation at small defects and the general surface background of the article being inspected and yet maintain the radioactivity at a level safe for human operators.

Briefly stated in accordance With one aspect of our invention, we provide a radioactive magnetic particle material having a level of radioactivity of about 001-100 rnillicuries per gram of material.

The subject matter which We regard as our invention is particularly pointed out and distinctly claimed in the concluding portion of this specification. Our invention, however, both as to organization and method of operation, together with further objects and advantages thereof, may be best understood by reference to the following detailed description.

The use of magnetic particles which have been irradiated allows our provision of an accurate mechanized magnetic particle inspection method. However, we believe that the level of radioactivity of the magnetic particles must be within the range of 0.01100 rnillicuries per gram of magnetic material in order to be at a proper level for detection and yet be controllably safe for use by human operators.

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The proper level of radiation varies among articles being inspected, the best level of radioactivity in a dispersion of magnetic materials in a fluid being determined experimentally in any given case. For example, if iron-59 is to be considered, the millicurie strength (mc.), necessary to obtain a sufficiently high contrast of radioactive magnetic iron particles attracted to a flaw to yield a count of statistically significant difference from a part without a flaw can be calculated using standard reactor irradiated units and well known statistical formulas.

Based on our knowledge of physics and our experience with conventional magnetic particle inspection, we made the following calculations. We concluded that with a millicurie strength of about 1.0 millicurie per gram (mc./ gm.) of material we could obtain a high contrast at controlled safety levels to the operator.

Our calculations were made using the specific activity reported from Oak Ridge Laboratories (OR) for iron-59 as being 0.15 mc./ gm. for the standard reactor irradiated unit. Since our material was irradiated at Brookhaven National Laboratory (B), the specific activity expected for irradiating a standard reactor unit was calculated as follows:

( p- )B=( pbnxjgi in which:

therefore 51.0 Ina/gm.

Since our material Was about 3 parts iron-oxide particles and 1 part mineral oil, and assuming 70% by weight, of the iron oxide is iron, then the expected activity per gram of paste, per standard reactor irradiated unit is:

gm. of iron gm. of paste 0'52O gm. of paste By definition:

l rnc.=2.22 X 10 disintegrations/ minute Therefore:

0.5 me lO disintegrations/ minute Assuming it is necessary to have counts above background to have statistically significant data, the theoretical limit of detection is:

100 e./m. 10 c./m.

Since the detection efliciency of the counter, for the geometry and radiation used, was about 10%, the practical limit of detection was about 10- grams of paste.

This means that, if the general background due to the residual activity on a good blade is about a 1000 c./m., and if there is at least an additional 10* grams of paste attracted to a defect, the difierence in counts between the two blades is statistically significant. Thus: a flaw or defect can be detected using our type of automatic mechanized method and apparatus.

In the following sample calculation using an iron oxide =10" gm. of paste paste, the paste was about 3 parts iron oxide particles and about 1 part carrier or surface active agent. Although in this calculation we mixed the iron oxide into a paste with the carrier, it should be understood that iron oxide powder alone can be used and in fact is preferred to a paste for irradiation. Assuming 70 percent by weight of the iron oxide is iron, then the expected activity per gram of material, per reactor irradiated unit which we used is 0.525 mc./ gm. of material. In order to determine if this amount of material can be of a statistically significant amount, the following calculations can be employed:

By definition one millicurie is equal to 222x disintegrations per minute. Therefore 0.5 millicurie is about equal'to '10 disintegrations per minute' The actual detection of a defect is based upon a significant statistical difference in count between a blade that is good and a blade with a rejectable flaw. A statistical formula for determining the confidence level (C. L.) of a significant difference between two counts obtained in equal counting periods for the level of radioactivity of articles is:

where N =count from one sample N count from a different sample N 25 T;0.674, there is significant ditference at the 50% CL. T;1.64, there is significant difference at the 95% CL. T5196, there is significant difference at the 95 C.L. T 52.58, there is significant difference at the 99% CL.

Although we have found, as we have shown above, that a statistically significant difference of the radioactivity could be produced and detected on an article between a flaw or defect and its background, we recognized that another important factor in our system was the amount of the irradiation necessary to produce material which would be safe with human operators. We have found that irradiated material within the range of 0.0l-100 mc./ gm. strength can be safely used with proper controls and shields. Although we have talked of materials in paste form, we have found that the use of powder alone not only substantially improves the process but the powder is more conveniently handled than the paste which has a tendency to cake during irradiation. In one of our tests for the preparation and checking of irradiated paste in a dispersion to determine whether or not it was safe for use with human operators, we had subjected 33.4 grams of paste, including 3 parts of iron oxide particles and one part of mineral oil, to neutron bombardment in an air cooled hole. This bombardment produced nuclear reactions with the elements in the paste resulting in the emission of detectable radiation. The irradiation process was continued intermittently for a total of 553.5 hours in a neutron flux of 3.1 10 thermal neutrons per square centimeter per second at a temperature of 50-75" C.

At the end of the irradiation period a dose rate of 1 roentgen per hour at a distance of 1 foot from the unshielded paste was measured. Thus the dose rate received by personnel with proper shielding was found to be less than the maximum amounts permitted by Atomic Energy Commission regulations at this time.

The paste, due to the heating of the organic carrier, which in this example was mineral oil, came from the reactor in cake form. However, We ground the cake into fine particles in an automatic mortar and pestle grinder, and dispersed the particles in kerosene. The measured dose rate at the surface of two inches of lead enclosing a beaker containing the dispersion was less than 1 milliroentgen per hour.

It was found that the magnetic properties of the paste were not affected by the nuclear irradiation.

The articles which we selected for test were blades such as for mom elastic fiuid flow apparatus; for example, gas turbine blades. The method which we used to detect flaws or defects was first to magnetize the blade longitudinally, passing about 1,000 amperes through a coil of eight turns to induce a magnetic field in the article. We then introduced, by dipping, the blades in the above described radioactive magnetic particle liquid dispersion to allow proper distribution of the particles on the surface of the blades. We then rinsed the blade with a bath of trichloroethylene to reduce excess radioactivity on the blade surface and accelerate drying. In some known magnetic particle inspection processes such a rinse tends to reduce accuracy rather than to increase accuracy as it does in our case. In this example we used trichloroethylene as a rinsing material.

After inspection, such as is described in the above identified co-pending application, articles which have been rejected and decontaminated may then be given to a skilled human operator for standard magnetic particle inspection so that he may more efliciently use his experience to judge if in fact the article is an unsatisfactory one. By elimina tion of the need for human operators in a large portion of magnetic particle inspection of articles, we have greatly improved the productivity of any system relying on such nondestructive tests.

Those skilled in the art of chemistry and physics will recognize the modifications and variations of which the present invention is capable.

What is claimed is:

1. A paste for use with water to form a dispersion for use in a magnetic particle inspection method comprising: radioactive iron oxide having a level of radioactivity of about 0.01- millicuries per gram of iron oxide; and an amount of a water soluble carrier suflicient to form a paste.

2. A paste for use with kerosene to form a dispersion for use in a magneitc particle inspection method comprising: radioactive iron oxide having a level of radioactivity of about 001-100 millicuries per gram of iron oxide; and an amount of a kerosene soluble carrier sufficient to form a paste.

OSCAR R. VERTIZ, CARL D. QUARFORTH,

Examiners,

4/1944 Mysels 252-2011 6/1959 Muffly 252 -3011 

1. A PASTE FOR USE WITH WATER TO FORM A DISPERSION FOR USE IN A MAGNETIC PARTICEL INSPECTION METHOD COMPRISING: RADIOACTIVE IRON OXIDE HAVING A LEVEL OF RADIOACTIVITY OF ABOUT 0.01-100 MILLICURIES PER GRAM OF IRON OXIDE; AND AN AMOUNT OF WATER SOLUBLE CARRIER SUFFICIENT TO FORM A PASTE. 